1. Field of the Invention
The present invention relates to a display apparatus that includes electroluminescence (hereinafter referred to as EL) elements and thin film transistors (hereinafter referred to as TFTs) which are fabricated on a substrate, and also to a method of fabricating the same.
2. Description of the Related Art
Recently, display apparatus using TFT elements and EL elements such as organic EL elements have been noted as display devices, in place of the CRTs (Cathode Ray Tubes) or LCD (Liquid Crystal Display) apparatus.
FIG. 1 is a cross sectional view illustrating a conventional display apparatus including organic EL elements and TFT elements. FIG. 1 shows a laminated structure in which an organic EL element is deposited over a TFT structure. The structure is formed as follows:
A gate electrode 2 is formed on a transparent insulating substrate 1 such as glass or synthetic resin. An insulating film 3 is formed on the gate electrode 2. An active layer 4 of polycrystalline silicon is formed on the insulating film 3. A source region 4s and a drain region 4d into which impurities are implanted are formed in the active layer 4. An interlayer insulating film 8 formed of a SiO2 film 6 and a SiN film 7 is formed on the active layer 4. The source region 11s is connected to the source electrode 10s via the contact hole 9 formed in the interlayer insulating film 8. The drain region 4d is connected to the drain electrode 10d via the contact hole 9 formed in the interlayer insulating film 8.
Planarization insulating film 11 is formed on the electrodes 10s and 10d and the interlayer insulating film 8. The source electrode 10s is connected to an anode electrode 28 of an organic EL element formed on a TFT element via the contact hole 12 formed in the planarization insulating film 11.
The organic EL element is formed by successively laminating an anode 2 formed of: a transparent electrode of ITO (Indium Tin Oxide); an organic layer comprised a second hole transfer layer 27 of MTDATA (4,4xe2x80x2-bis(3-methylphenylphenylamino)biphenyl), a first hole transfer layer 26 of TPD (4,4xe2x80x2,4xe2x80x3-tris(3-methylphenylphenylamino)triphenylamine), a luminous layer 25 of Bebq2(10-benzo[h]quinolinol-beryllium complex) containing Quinacridone derivative, and an electron transfer layer 24 of Bebq2; and a cathode electrode 23 of magnesium and indium alloy (MgIn). The organic layer is made of an organic chemical compound. Thus, an EL element is constructed the organic layer, the anode electrode 28, and the cathode electrode 23.
In the organic EL element, the holes injected from the anode electrode 28 and the electrons injected from the cathode electrode 23 are recombined inside the luminous layer 25. Excitons are generated by exciting the organic molecules of the luminous layer 25. The luminous layer 25 radiates light through the process of the excitons disappearing. The light is radiated out from the transparent anode electrode 28 through the transparent insulating substrate 1.
However, in the conventional display apparatus structure, since the organic EL element emits light through the side of the substrate 1 on which TFT elements are formed, the TFT structure blocks the emitted light so that the display pixel aperture ratio cannot be increased.
Furthermore, since the TFT elements must be small-sized to the extent that the luminous light is not blocked, there are severe limitations on increasing the size of each TFT element as well as the TFT element capability.
The present invention is made to solve the above-mentioned problems involved in the conventional display apparatus. It is an object of the invention to provide a display apparatus that can improve the display pixel aperture ratio and can increase the degree of freedom in deciding the size and the drive capability of a thin film transistor which drives an EL element.
According to the present invention, the display apparatus comprises: a substrate; thin film transistors formed on the substrate, each of the thin film transistors having a source electrode and a drain electrode; and electroluminescence elements respectively formed over the thin film transistors, each of the electroluminescence elements having a cathode electrode, an anode electrode, and a luminous layer formed between the cathode electrode and the anode electrode; wherein each of the electroluminescence elements emits toward the reverse side of the substrate.
Each of the electroluminescence elements comprises the cathode electrode, the luminous layer and the anode electrode successively formed above the thin film transistor. The cathode electrodes is connected to a source or drain electrode of the corresponding thin film transistor. Moreover, each of the thin film transistors drives the corresponding electroluminescence element.
Since light is emitted from the reverse side of the substrate, the thin film transistor formed on the substrate side does not block the light, so that the aperture ratio can be increased.
It is not required to miniaturize the thin film transistor to the extent that the light is not shielded. Hence, the thin film transistor can be designed with high freedom of size. Thin film transistors with high performance can be formed without constraints in size.
According to the present invention, the electroluminescence element is constructed by successively forming a cathode electrode, a luminous layer, and an anode electrode over the thin film transistor.
The anode electrode is made of a metal material and can cover over only a part of the display pixel area within a unit display pixel area.
The above-mentioned planar structure can externally emit light from the reverse side of the substrate, that is, the anode side. Moreover, the anode electrode in, for example, a comblike, meshlike or gridlike form can externally emit a sufficient amount of light.
The anode electrode of the electroluminescence element can be formed by a vapor evaporation process. Furthermore, according to the present invention, the display apparatus is fabricated through the steps of forming the thin film transistors on a substrate; forming an insulating film to cover the thin film transistor; forming contact hole at predetermined position of the insulating film, and then forming the cathode electrode of each of the electroluminescence elements to respectively make contact with a source electrode or a drain electrode of the thin film transistor via the holes; forming said luminous layer over the cathode electrode; and forming an anode electrode over the luminous layer using an opaque metal material through a vapor evaporation method. The anode electrode is preferably formed to partially occupy a unit display pixel region.
The electroluminescence element comprises an organic electroluminescence element using an organic material for the luminous layer.